An OS project is hugely unlikely to ever get into the "useful" stage (as in, actually working, and a good choice to actually work with). Kernel space programming is also a very involved, unforgiving environment, with many things you need to do / learn for it that you will never, ever, need in any other field (except hardware driver programming). It involves handling lots of technical information. You are also, by definition, reinventing the wheel.
For some, that's exactly the incentive why they pick it up.
To be answering your question more directly, if you check the Wiki articles
Required Knowledge and
Beginner Mistakes, you will find that they strongly
discourage "learning programming" while doing OSDev. An operating system is the most complex piece of software you can imagine,
and done in a more-hostile-than-average environment.
Again, some take that as a challenge to do it anyway, but personally, I feel it is just setting yourself up for frustration.